ediff-narrow-control-frame-leftward-shift

ediff-narrow-control-frame-leftward-shift is a variable defined in `ediff-wind.el'.
Its value is

3

Documentation:
The leftward shift of control frame from the right edge of buf A's frame.
Measured in characters.
This is used by the default control frame positioning function,
`ediff-make-frame-position' to adjust the position of the control frame
when it shows the short menu. This variable is provided for easy
customization of the default.

You can customize this variable.